This invention relates to tufting machines and more particularly to a knife block for supporting a pair of knives adapted to cooperate with and sever yarn loops on respective adjacent hooks positioned at different elevations relatively to the base fabric and thereby form cut pile of different pile heights in adjacent rows of stitching.
In co-pending United States patent application of Wilson Ser. No. 048,611, filed June 13, 1979, assigned to the assignee of the present invention, apparatus and method are disclosed for producing a tufted fabric having in alternate rows of stitching selective cut pile and loop pile and wherein the cut pile in alternate rows are of different pile height and all the loop pile is of a third and lower pile height. In the machine there disclosed a first set of needles penetrates the base fabric to cooperate with a first set of hooks at a first depth, and a second set of needles penetrates the backing fabric to cooperate with a second set of hooks at a second and lower depth. Each hook includes a spring clip and a respective cooperating knife and a yarn feed pattern attachment selectively feeds yarn to the first set of needles at a high and a low feed rate and to the second set of needles at an intermediate and the low feed rate. The low feed rate affects back-drawing of yarn past the spring clip to form loop pile while the two other feed rates allow the yarn to stay on the hooks to be severed by the respective knives.
With this system adjacent knives are mounted in a common knife block but coact with respective elevationally spaced hooks. Thus, the knives carried by a common knife block extend elevationally to different levels. It has now been determined that since the knives act at a small angle against a ledger edge of the respective hooks to provide a scissors-like cutting action, an unequal pressure or tension on adjacent hooks and an unequal resiliency of the knives exists when knife blocks of conventional construction are used in this environment. This in turn results in unequal wear on adjacent knives and inconsistent loop cutting action.